1) Field of the Invention
The present invention relates to a continuous process for preparing rubber modified high impact resins.
More specifically, the present invention relates to the process in which, when a starting solution obtained by dissolving a rubber-like polymer in a starting monomer containing an aromatic vinyl monomer or a mixture consisting of an aromatic vinyl monomer and a vinyl cyanide monomer is subjected to a continuous polymerization reaction by using a bulk polymerization process or solution polymerization process, the rubber-like polymer used is brought into a desired particle size dispersed in the reacting liquid.
2. Description of the Prior Art
High impact polystyrene resins (hereinafter abbreviated as HI-PS resin), which are obtained by polymerizing styrene in the presence of a rubber-like polymer, are a kind of polystyrene resin having improved impact strength and are widely used in various application fields. Recently, the method for producing the HI-PS resin, is mainly a continuous bulk polymerization process although the resin is partially prepared still by a batchwise polymerization using a bulk or suspension polymerization process.
Such continuous method has heretofore been widely used for preparing the rubber modified styrene resin in which the rubber-like polymer is dispersed in the resin as particles having an average particle size of from 0.1 to 10.mu.. Since the particle size of the rubber-like polymer in the resin produced gives important influences on the resin properties such as impact strength and surface gloss, the operating process for controlling the particle size of the rubber-like polymer in the resin holds an important position in the techniques for preparing the rubber modified polystyrene resins.
Generally, the particle size and the particle form of the rubber-like polymer dispersed in the rubber modified styrene resin are settled depending on operating conditions in the period of time of the polymerization reaction, in which period the reacting liquid containing the rubber-like polymer in the course of the polymerization reaction varies from the state of a continuous phase into the state of an interrupted phase, namely the operation conditions for forming the dispersed rubber particles (this operation is hereinafter referred to as "particle forming operation" in the present invention). A continuous preparation method of the rubber modified styrene resins, is more difficult than a continuous preparation method of ordinary styrene resins containing no rubber-like polymer or a batchwise method of preparing a rubber modified styrene resins, because in the continuous preparation method of the rubber modified styrene resins, the particle forming operation is conducted under the conditions accompanying the transfer operation of the reacting liquid from one reactor to another and the polymerization reaction. Therefore, some processes which are complicated have heretofore been proposed for solving such difficulty. For example, a process, in which a preliminary polymerization referred to as the preliminary grafting of the rubber-like polymer is performed prior to the particle forming operation, has been proposed in Japanese Patent Publication No. 29793/1977. A process, in which the polymerizing solution has already reached a monomer conversion in the range of from 30 to 80 percent is mixed with the starting monomer to be polymerized, has been proposed in Japanese Patent Laid-open No. 7794/1978. In Japanese Patent Publication No. 17725/1984, a process in which a part of the polymerizing solution is recycled to the starting monomer system, has also been proposed.
It has also been well known that a procedure of strongly stirring the polymerizing liquid in the early polymerization stage wherein the conversion of the monomer is low, is effective as an operating manner for changing the continuous phase containing the rubber-like polymer (hereinafter abbreviated as "rubber phase") to rubber particles dispersed in the polymerizing liquid. It has also been known in Japanese Patent Publication No. 9245/1987 that a polymerization reactor of the stirred vessel type is effective in such step of the particle forming operation. Although it is simple, it requires a large amount of agitation power for forming the particles having a particle size of 0.6.mu. or smaller.
So-called ABS resins which are obtained by polymerizing styrene and acrylonitrile in the presence of a rubber-like polymer have various applications because of their excellent impact strength, chemically resistant properties, heat resistant properties, stiffness, surface gloss and the like. ABS resin is generally produced in a so-called emulsion polymerization process in which a latex containing a rubber-like polymer component is added to monomers of styrene and acrylonitrile, and then the mixture thus obtained is polymerized. Generally, the emulsion polymerization process has such problems that the facilities for its polymerization step become larger by the reason of using a latex in an amount of several times of the polymer, and that the control of its whole procedure becomes complicated because various steps such as emulsifying step, coagulating step and drying step, are essentially required in the process, and, moreover, the resins thus obtained are contaminated with impurities such as emulsifiers, coagulants which inevitably be used in the process. As a manner for improving such emulsion polymerization process, there is proposed a method in Japanese Patent Publication Nos. of 35354/1974 and 35355/1974 that, after extracting the rubber component as is in the latex to be used into the monomers of styrene and acrylonitrile to be polymerized, the monomers containing the rubber component are then polymerized by using a continuous bulk polymerization process to produce ABS resin. Although the procedure of this process is considerably simplified in comparison with a conventional emulsion polymerization process, the complicated extraction step remains in the process.
As other processes for preparing ABS resin, some continuous bulk polymerization processes or solution polymerization processes have been proposed in Japanese Patent Publication No. 20303/1970, and Japanese Patent Laid-open Nos. 9144/1972 and, 36201/1980. These processes have merits that the steps of polymerization and the after treatment in the process are simplified and that the amount of waste substances, which are to be wasted and will become a cause of environmental pollution, is decreased. However, these processes had such problems that the physical properties of the resins prepared by using such processes are not always excellent and especially one of the remarkable feature of ABS, the surface gloss of the molded resins, is inferior and that a special equipment is required in the process.
In the preparation of ABS resins by using a continuous bulk polymerization process or solution polymerization process, the particle size of the rubber-like polymer dispersed in the resin gives a remarkable influence on the resulting resin in physical properties such as impact strength, surface gloss and the like. Therefore, the operating manner for controlling the particle size of the rubber-like polymer in the resins holds a important position in the techniques for preparing ABS resins. It is well known that a process of strongly stirring the polymerizing liquid, which is in the early polymerization stage having a low conversion of the monomer used, is effective as an operating manner for changing the continuous phase containing the rubber-like polymer (hereinafter abbreviated as "rubber phase") to the rubber particles dispersed in the polymerizing liquid. The present inventors have also proposed such a manner in Japanese Patent Publication No. 2284/1988 that a reactor of stirred vessel type is used in such a process stage. Although this process is simple, the process requires a very large amount of agitation power for making the rubber particles having a particle size of 0.8.mu. or smaller.
Japanese Patent Publication No. 18477/1974 discloses that the rubber particles having a desired size can be obtained by applying a shearing treatment with a particle dispersing machine to rubber particles previously generated in the polymerizing liquid. In this manner, after the shearing treatment of the rubber particles with the dispersing machine, the cross-linking reaction of the rubber-like polymer and the polymerization reaction of the whole monomers are completed concurrently by adding a cross-linking agent to the polymerizing liquid containing the sheared rubber particles and by maintaining the polymerizing liquid at a temperature in the range of from 150.degree. to 200.degree. C.
However, it is recently desired to improve the method of the particle forming operation in the continuous production processes of the rubber modified high impact resins such as HI-PS resins and ABS resins, for answering the demand of the market which desires higher physical properties of such resins due to the broadened application of such resins and for answering to the increased need of lowering the production cost of such resins by using a more efficient preparing process.